Obtaining condition for the balance Wheatstone bridgeApplying Kirchoff's loop rule to closed loop ADBA $-I_1 R_1+0+I_2 R_2=0 (I_g=0)$ .......(i) For loop CBDC $-I_2 R_4+0+I_1 R_3=0$ .......(ii) $\Rightarrow$ from equation (i) $\;\frac{I_1}{I_2}=\;\frac{R_1}{R_2}$ From equation (ii) $\;\frac{I_1}{I_2}\;=\;\frac{R_4}{R_3}$ $\therefore \;\; \;\frac{R_1}{R_2}\;=\;\frac{R_4}{R_3}$